1. Field of the Invention
The present invention relates to an interchangeable lens camera system. More particularly, the invention relates to a camera system which can correct the focus detection and the photomerry in accordance with variations or changes with time in optical properties of interchangeable lenses.
2. Related Background Art
Conventionally known is an interchangeable lens camera system provided with a focus detecting apparatus which can detect a defocus amount between an imaging plane of photographic lens and a film plane by detecting phases of a pair of images formed by a pair of beams coming from two different regions in exit pupil of interchangeable lens. Also known is an interchangeable lens camera system provided with a focus detecting apparatus for carrying out the above focus detection at a plurality of positions in screen.
Such systems need to correct the defocus amount detected in accordance with various aberrations of lens, because an F-value of a beam forming an actual image is different from that of a beam for focus detection. The correction of defocus amount was carried out in the systems such that each interchangeable lens stored lens data according to designed aberrations thereof and that the lens data was read into the camera body for correction. In case that a focus detection position is off the optical axis, an image plane where the focus is detected is curved, so that there is a relative error between focus detection positions off and on the optical axis. Therefore, lens data for correction was calculated from designed aberrations in accordance with a distance to the optical axis and stored for each interchangeable lens. Lens data of each lens was read into the camera body for correction.
Another focus detection error, which was caused by a difference between the spectral property of film speed and the spectral property of sensor for focus detection, was also corrected in the arrangement in which each interchangeable lens stored lens data according to designed infrared aberrations thereof and in which the lens data was read into the camera body for correction.
Further, there is known another interchangeable lens camera system provided with a metering device for carrying out the photometry in screen through the interchangeable lens. In this system an interchangeable lens stores as lens data information on transmittance, F-value at full open aperture and exit pupil position thereof and the lens data is read into the camera body when a metering value is calculated from an output of the metering sensor.
The focus adjusting apparatus as described, however, had the following problems.
Since the lens data is common to all same interchangeable lenses, the conventional methods cannot absorb individual variations of optical properties such as aberrations caused by eccentricity of lens, a variation of lens spacing, a variation of index of refraction of lens, or the like, which could be produced in assembly of lenses; a spectral transmittance; an F-value at full open aperture; and an exit pupil position, inevitably making an error.
The individual variations of optical properties can be permissible only as to photography if they are within a certain range, but they cannot be always permissible for metering or focus detection. Mechanical adjustment could be conducted to cancel the variations of optical properties so as not to cause a final error in photometry or in focus detection, which requires a lot of time and costs.
In addition, the conventional methods failed to deal with not only a change in optical properties after an interchangeable lens is disassembled and then reassembled for repair, but also a change with time in optical properties due to environmental changes such as the temperature and the humidity. Thus, an error is inevitably made in focus detection or in photometry.
For example, if the focus detection is carried out at the center of screen using a focus detection optical system of split-pupil re-imaging method, a beam for focus detection is one with a large F-value relatively near the optical axis, but a beam for exposure upon open aperture photography is one with a small F-value. Thus, it is known that there is a difference H caused between the focus detection image plane P and the best image plane Q because of influence of spherical aberration A, as shown in FIG. 9. In FIG. 9 the vertical axis represents a height of incident light and the horizontal axis a position in the direction of optical axis, on which R is a position of Gaussian image plane.
The position of best image plane Q can be calculated by the following conventional method. Since the difference H between the two image planes as described above varies depending upon the type of lens, a value of difference H is preliminarily calculated from a designed spherical aberration and fixedly stored as lens data in mass production in a ROM built in each lens. When each lens is mounted on the camera body, the lens data is read from the lens into the body side. The position of focus detection image plane P detected is corrected by the difference H from the lens data to obtain the position of best image plane Q.
Lenses of the same type could have, however, an individual variation or a change with time of spherical aberration. For example, if the spherical aberration is as shown by A' in FIG. 9, the focus detection image plane is changed to P' and the best image plane to Q', whereby the difference therebetween is changed to H'. In case that the correction is made by the designed difference H ignoring this change, there would be an error H--H' between the best image plane Q' of lens and the calculated best image plane Q upon correction, as shown in FIG. 10.
Additionally, if the focus detection is carried out at the periphery of screen in the focus detection optical system of split-pupil re-imaging method, the difference H between the focus detection image plane P and the best image plane Q also changes into Hm (g) and Hs (g) depending upon a distance g to the optical axis because of the curvature of field, as shown in FIG. 11. In FIG. 11 the vertical axis represents a distance g to the optical axis and the horizontal axis is a distance in the direction of optical axis. Hm denotes an image plane difference when a focus detection region is located on a radial line with the center on the optical axis, and Hs an image plane difference when the focus detection region is located on a line perpendicular to the radial line.
The following method is known for correction of image plane difference. The above image plane difference is preliminarily calculated from optical design data for each lens type for example as coefficients in polynomials of distance g representing Hm (g) and Hs (g), or as values of image plane difference at predetermined distances g1 and g2. Then, the calculated image plane difference data is fixedly stored as lens data in mass production in a ROM built in lens. When a lens is mounted on the body, the stored lens data is read from the lens into the body. A position of focus detection image plane detected on the body side is corrected in accordance with the lens data, the focus detection position and the arrangement of detection regions. Then, the position of best image plane is calculated with the corrected value.
In FIG. 12 the vertical axis represents a distance to the optical axis and the horizontal axis a position in the direction of optical axis. A solid dot ".cndot." stands for a position of corrected image plane when the focus detection region is located on a radial line, and a blank square ".quadrature." for a position of corrected image plane when the focus detection region is located on a line perpendicular to the radial line. Aligned on the vertical axis are positions of corrected image planes at distances X, O and --X to the optical axis.
Lenses of the same type could have, however, individual variations or changes with time of the above image plane differences Hm (g) and Hs (g). If a lens has eccentricity for example, the image plane differences become asymmetric with respect to the optical axis, as shown as Hm' (g) and Hs' (g) in FIG. 13. If the correction is carried out with the designed image plane differences Hm (g) and Hs (g) ignoring this change, errors Hm (g)-H'm (g) and Hs (g)-H's (g) will be made between the best image plane Q' of lens and the calculated best image plane Q after correction, as shown in FIG. 14. Therefore, the corrected image plane positions cannot be aligned at distances X, O and --X to the optical axis.